Recovery of bis(diarylphenol) ligands during the production of isopulegol

ABSTRACT

The present invention relates to a method of working up an aluminium-containing reaction product from the production of isopulegol by cyclization of citronellal in the presence of complex compounds, comprising at least one ligand of the formula (I), 
     
       
         
         
             
             
         
       
         
         where Ar 1 , Ar 2 , Ar 3 , Ar 4  are chosen from C 6 -C 15 -aryl or C 2 -C 15 -hetero; R 1 , R 2 , R 3 , R 4  are chosen from H, C 1 -C 6 -alkyl, C 1 -C 6 -perfluoroalkyl, C 1 -C 6 -alkoxy, C 7 -C 12 -aralkyl, halogen, SiR 5b R 6b R 7b , C 6 -C 10 -aryl, NR 8b R 9b , SR 10b , NO 2 ; and where R 1  or R 2  and/or R 3  or R 4 , together with A, can form an aromatic or nonaromatic cycle; etc.; 
         in which a) the aluminum-containing reaction product is subjected to distillative separation, b) the isopulegol-depleted bottom product is brought into close contact with an aqueous base and c) the ligand of the formula (I) is separated off from the organic phase, preferably by crystallization. 
       
    
     Moreover, the invention relates to a method of producing isopulegol, and to a method of producing menthol.

The present invention relates to a method for working up an aluminum-containing reaction product from the production of isopulegol by cyclization of citronellal in the presence of complex compounds, comprising at least one ligand of the formula (I).

Moreover, the invention relates to a method for producing isopulegol, and to a method for producing menthol.

In terms of amount, menthol is the most important aroma chemical worldwide. The demand for menthol continues to be covered largely by isolation from natural sources. In addition, however, there are also synthetic routes to menthol, sometimes in racemic form, sometimes in the form of the natural enantiomer L-menthol.

An important intermediate for producing racemic such as optically active menthol is isopulegol, which is usually produced by a cyclizing oxo-ene reaction of citronellal in the presence of Lewis-acidic catalysts and is usually produced in the form of mixtures of the four diastereomers isopulegol, iso-isopulegol, neo-isopulegol and neoiso-isopulegol.

Suitable catalysts which have been described for carrying out the abovementioned cyclization of citronellal to isopulegol are both heterogeneous catalysts, such as SiO₂, Al₂O₃/SiO₂, SiO₂/ZrO₂, SiO₂/TiO₂ mixed catalysts, moredenites, faujasites, montmorillonites and zeolites—and also homogeneous catalysts, such as, for example, sulfonic acids or Lewis acids, such as, for example, SnCl₄, ZnCl₂ or ZnBr₂.

EP-A 1 225 163 describes the cyclization of citronellal to isopulegol in the presence of tris(2,6-diphenylphenol) aluminum catalysts. This method for cyclizing citronellal to isopulegol uses catalyst complexes which are expensive and can only be produced with complexity. After the described method, to be carried out in a homogeneous phase, the catalyst complex is hydrolyzed when the reaction is complete. Possible recovery and reusability of the ligand liberated in the process is not described.

By contrast, the unpublished PCT/EP 2006/060416 describes bis(diarylphenoxy)aluminum compounds which are obtainable by reacting a bis(diarylphenol) ligand of the formula (I) with a suitable aluminum compound, and methods of producing isopulegol and menthol in the presence of these compounds. Here, processes are also disclosed which permit recovery of the bis(diarylphenol) ligands of the formula (I) used. Recovery takes place by crystallization from a bottom product obtained during the distillative separation of isopulegol from a reaction product of the cyclization of citronellal. However, such a work-up leads to yields and purities which are not entirely satisfactory, especially in the case of a continuous method for producing isopulegol.

Accordingly, it was an object of the present invention to provide a method which, after cyclization of citronellal to isopulegol has taken place, permits the bis(diarylphenol) ligands used to be recovered and reused with improved purity and yield. Specifically, it was the aim to facilitate a continuous method with good space-time yield.

Surprisingly, it has now been found that the bis(diarylphenol) ligands used can be recovered in improved purity and yield after cyclization of citronellal by distillative separation of the aluminum-containing reaction product to give an isopulegol-enriched top product and an isopulegol-depleted bottom product, then bringing the isopulegol-depleted bottom product into contact with an aqueous base to give an aluminum-containing aqueous phase and an organic phase comprising the majority of the ligands of the formula (I) and then separating off the ligand of the formula (I) from the organic phase, preferably by crystallization.

The present invention thus provides a method for working up an aluminum-containing reaction product from the production of isopulegol by cyclizing citronellal, comprising

i) isopulegol, ii) at least one ligand of the formula (I),

-   -   where     -   Ar¹, Ar², Ar³, Ar⁴, independently of one another, are chosen         from C₆-C₁₅-aryl radicals or C₂-C₁₅-heteroaryl radicals, which,         if appropriate, can in each case carry 1 to 7 identical or         different substituents chosen from C₁-C₆-alkyl,         C₁-C₆-perfluoroalkyl, C₁-C₆-alkoxy, C₇-C₁₂-aralkyl, halogen,         SiR^(5a)R^(6a)R^(7a), optionally substituted C₆-C₁₀-aryl,         NR^(8a), R^(9a), SR^(10a), NO₂,     -   R¹, R², R³, R⁴, independently of one another, are chosen from         hydrogen, C₁-C₆-alkyl, C₁-C₆-perfluoroalkyl, C₁-C₆-alkoxy,         C₇-C₁₂-aralkyl, halogen, SiR^(5b)R^(6b)R^(7b), optionally         substituted C₆-C₁₀-aryl, NR^(8b)R^(9b), SR^(10b), NO₂ and where         -   R¹ or R² and/or R³ or R⁴, together with A, can form an             aromatic or nonaromatic cycle, and     -   A is a straight-chain or branched and/or cyclic hydrocarbon         radical having 1 to 25 carbon atoms which may be saturated or         mono- or polyunsaturated and/or partially aromatic and can, if         appropriate, have one or more identical or different heteroatoms         chosen from O, S, NR¹¹, and/or one or more identical or         different functional groups chosen from the functional groups         C(O), S(O), S(O)₂ and can, if appropriate, carry one or more         identical or different substituents chosen from the substituents         C₁-C₆-alkyl, C₁-C₆-perfluoroalkyl, C₁-C₆-alkoxy, C₁-C₁₀-acyloxy,         C₇-C₁₂-aralkyl, halogen, —SiR^(5c)R^(6c)R^(7c), optionally         substituted C₆-C₁₀-aryl, substituted or unsubstituted         C₂-C₁₀-hetaryl, NR^(8c)R^(9c), SR^(10c), NO₂, C₁-C₁₂-acyl,         C₁-C₁₀-carboxyl, or         -   is a C₆-C₁₅-aryl radical or a C₂-C₁₅-heteroaryl radical             which can, if appropriate, in each case carry 1 to 5             substituents chosen from C₁-C₆-alkyl, C₁-C₆-perfluoroalkyl,             C₁-C₆-alkoxy, C₇-C₁₂-aralkyl, halogen, SiR^(5d)R^(6d)R^(7d),             substituted or unsubstituted C₆-C₁₀-aryl, NR^(8d), R^(9d),             SR^(10d), NO₂, or         -   is a functional group or a heteroatom chosen from the group             —O—, —S—, —N(R¹¹)—, —S(O)—, —C(O)—, —S(O)₂—, —P(R¹¹)—,             —(R¹¹)P(O)— and —Si(R¹²R¹³),         -   where the radicals R^(5a), R^(6a), R^(7a), R^(8a), R^(9a),             R^(10a) to R^(5d), R^(6d), R^(7d), R^(8d), R^(9d), R^(10d),             R¹¹, R¹² and R¹³ are in each case independently of one             another chosen from C₁-C₆-alkyl, C₇-C₁₂-aralkyl and/or             substituted or unsubstituted C₆-C₁₀-aryl and where the             radicals R^(8a) and R^(9a), R^(8b) and R^(9b), R^(8c) and             R^(9c), R^(8d) and R^(9d), independently of one another, can             in each case together also form a cyclic hydrocarbon radical             having 2 to 8 carbon atoms which can have one or more             identical or different heteroatoms chosen from the group O,             S, NR^(11a), and R^(11a) can have the meanings given for             R¹¹,         -   in free and/or complex-bound form,             in which

-   a) the aluminum-containing reaction product is subjected to     distillative separation to obtain an isopulegol-enriched top product     and an isopulegol-depleted bottom product,

-   b) the isopulegol-depleted bottom product is brought into close     contact with an aqueous base to give an aluminum-containing aqueous     phase and an organic phase comprising the majority of the ligands of     the formula (I),

-   c) the ligands of the formula (I) are separated off from the organic     phase.

In one preferred embodiment of the method according to the invention, the ligand of the formula (I) is separated off from the organic phase by crystallization.

The bis(diarylphenol) ligands of the formula (I) obtained by the method according to the invention can usually be converted to the reactive catalyst complex without further purification steps, within the scope of a new batch with the corresponding aluminum compounds of the formulae (II) or (III), as defined below, with no or no noteworthy decrease in the reactivity being established with catalyst complexes recovered in this way.

The bis(diarylphenol) ligands of the formula (I) have two phenol systems which in each case are substituted in both ortho positions relative to the phenolic hydroxy group by aromatics or heteroaromatics (Ar¹ to Ar⁴) and are joined together via a structural element A and can, if appropriate, also carry further substituents (R¹ to R⁴).

The aromatic or heteroaromatic substituents Ar¹ to Ar⁴ may, independently of one another, be identical or different. Preferably, the two substituents bonded in each case to a phenol system (Ar¹ and Ar² or Ar³ and Ar⁴) are pairwise identical. Particularly preferably, all four substituents Ar¹ to Ar⁴ are identical.

The specified substituents Ar¹ to Ar⁴ are aryl radicals having 6 to 15, preferably 6 to 10, carbon atoms or heteroaryl radicals having 2 to 15, preferably 3 to 10, carbon atoms in the aromatic ring system. Aryl radicals having 6 to 15 carbon atoms are, for example, phenyl, naphthyl, anthracenyl, preferably phenyl and naphthyl.

The specified heteroaryl radicals having 2 to 15 carbon atoms have 1 to about 6, generally 1 to 3, identical or different heteroatoms which are chosen from the group of heteroatoms O, S and N. Examples thereof which may be mentioned are the following heteroaryl radicals: 2-furyl, 3-furyl, 2-thienyl, 3-thienyl, 2-pyrrolyl, 3-pyrrolyl, 3-isoxazolyl, 4-isoxazolyl, S-isoxazolyl, 3-isothiazolyl, 4-isothiazolyl, 5-isothiazolyl, 3-pyrazolyl, 4-pyrazolyl, 5-pyrazolyl, 2-oxazolyl, 4-oxazolyl, 5-oxazolyl, 2-thiazolyl, 4-thiazolyl, 5-thiazolyl, 2-imidazolyl, 4-imidazolyl, 1,2,4-oxadiazol-3-yl, 1,2,4-oxadiazol-5-yl, 1,2,4-thiadiazol-3-yl, 1,2,4-thiadiazol-5-yl, 1,2,4-triazol-3-yl, 1,3,4-oxadiazol-2-yl, 1,3,4-thiadiazol-2-yl and 1,3,4-triazol-2-yl, 2-pyridinyl, 3-pyridinyl, 4-pyridinyl, 3-pyridazinyl, 4-pyridazinyl, 2-pyrimidinyl, 4-pyrimidinyl, 5-pyrimidinyl, 2-pyrazinyl, 1,3,5-triazin-2-yl and 1,2,4-triazin-3-yl, 2-furyl, 3-furyl, 2-thienyl, 3-thienyl, 2-pyrrolyl, 3-pyrrolyl, 3-isoxazolyl, 4-isoxazolyl, 5-isoxazolyl, 3-isothiazolyl, 4-isothiazolyl, 5-isothiazolyl, 3-pyrazolyl, 4-pyrazolyl, 5-pyrazolyl, 2-oxazolyl, 4-oxazolyl, 5-oxazolyl, 2-thiazolyl, 4-thiazolyl, 5-thiazolyl, 2-imidazolyl, 4-imidazolyl, 1,2,4-oxadiazol-3-yl, 1,2,4-oxadiazol-5-yl, 1,2,4-thiadiazol-3-yl, 1,2,4-thiadiazol-5-yl, 1,2,4-triazol-3-yl, 1,3,4-oxadiazol-2-yl, 1,3,4-thiadiazol-2-yl and 1,3,4-triazol-2-yl, 2-pyridinyl, 3-pyridinyl, 4-pyridinyl, 3-pyridazinyl, 4-pyridazinyl, 2-pyrimidinyl, 4-pyrimidinyl, 5-pyrimidinyl, 2-pyrazinyl, 1,3,5-triazin-2-yl and 1,2,4-triazin-3-yl, benzofuryl, isobenzofuryl, benzothienyl, indolyl, isoindolyl, carbazolyl, pyridyl, quinolyl, isochinolyl and pyrazyl. Preferred heteroaryl radicals are, for example: 2-furyl, 2-pyridyl, 2-imidazoyl.

The aryl or heteroaryl radicals specified above for Ar¹ to Ar⁴ can, in each case independently of one another, be unsubstituted or carry 1 to about 7, preferably 1 to 3, in particular 1 or 2, identical or different substituents which are chosen from the group of substituents: C₁-C₆-alkyl, C₁-C₆-perfluoroalkyl, C₁-C₆-alkoxy, C₇-C₁₂-aralkyl, halogen, —SiR^(5a)R^(6a)R^(7a), substituted or unsubstituted C₆-C₁₀-aryl, —NR^(8a)R^(9a), —SR^(10a), —NO₂, where the radicals R^(5a), R^(6a), R^(7a), R^(8a), R^(9a), R^(10a) and R¹¹ to R¹³, in each case independently of one another, are C₁-C₆-alkyl, C₇-C₁₂-aralkyl and/or substituted or unsubstituted C₆-C₁₀-aryl, and the radicals R^(8a) and R^(9a), independently of one another can in each case together also form a cyclic hydrocarbon radical having 2 to 8 carbon atoms which can have one or more identical or different heteroatoms chosen from the group O, S and NR^(11a), and Linda can have the meanings given for R¹¹.

In this connection, the specified substituents within the scope of the overall present invention have the meanings given below by way of example:

C₁-C₆-alkyl such as, for example, methyl, ethyl, propyl, 1-methylethyl, butyl, 1-methylpropyl, 2-methylpropyl, 1,1-dimethylethyl, pentyl, cyclopentyl, 1-methylbutyl, 2-methylbutyl, 3 methyl butyl, 2,2-dimethylpropyl, 1-ethylpropyl, hexyl, cyclohexyl, 1,1-dimethylpropyl, 1,2-dimethylpropyl, 1-methylpentyl, 2-methylpentyl, 3-methylpentyl, 4-methylpentyl, 1,1-dimethylbutyl, 1,2-dimethylbutyl, 1,3-dimethylbutyl, 2,2-dimethylbutyl, 2,3-dimethylbutyl, 3,3-dimethylbutyl, 1-ethylbutyl, 2-ethylbutyl, 1,1,2-trimethylpropyl, 1,2,2-trimethylpropyl, 1-ethyl-1-methylpropyl and 1-ethyl-2-methylpropyl; C₁-C₆-perfluoroalkyl, such as, for example, trifluoromethyl, pentafluoroethyl, heptafluoropropyl, heptafluoroisopropyl, nonafluorobutyl; C₁-C₆-alkoxy, such as, for example, methoxy, ethoxy, propoxy, 1-methylethoxy, butoxy, 1-methylpropoxy, 2-methylpropoxy and 1,1-dimethylethoxy, pentoxy, 1-methylbutoxy, 2-methylbutoxy, 3-methoxylbutoxy, 1,1-dimethylpropoxy, 1,2-dimethylpropoxy, 2,2-dimethylpropoxy, 1-ethylpropoxy, hexoxy, 1-methylpentoxy, 2-methylpentoxy, 3-methylpentoxy, 4-methylpentoxy, 1,1-dimethylbutoxy, 1,2-dimethylbutoxy, 1,3-dimethylbutoxy, 2,2-dimethylbutoxy, 2,3-dimethylbutoxy, 3,3-dimethylbutoxy, 1-ethylbutoxy, 2-ethylbutoxy, 1,1,2-trimethylpropoxy, 1,2,2-trimethylpropoxy, 1-ethyl-1-methylpropoxy and 1-ethyl-2-methylpropoxy, C₇-C₁₂-aralkyl, such as, for example, benzyl, 1-phenylethyl, 2-phenylethyl; C₁-C₁₀-acyloxy, such as, for example, acetyloxy, propionyloxy; C₁-C₁₀-carboxyl, such as, for example, methoxycarbonyl, ethoxycarbonyl, propyloxycarbonyl, isopropyloxycarbonyl; C₁-C₁₀-acyl, such as, for example, formyl, acetyl, propionyl.

The expression “substituted or unsubstituted C₆-C₁₀-aryl” within the meaning of the present invention is aryl radicals which, as specified above, have one or more, generally 1 to about 3, identical or different substituents, where the substituents may be chosen, for example, from C₁-C₆-alkyl, C₁-C₆-perfluoroalkyl, C₁-C₆-alkoxy, C₇-C₁₂-aralkyl, halogen, silyl, dialkylamino and nitro.

Within the scope of the present invention, the term “halogen” is fluorine, chlorine, bromine and iodine, preferably fluorine and chlorine.

Within the scope of the present invention, the substituents —SiR^(5a)R^(6a)R^(7a) to —SiR^(5d)R^(6d)R^(7d) are in each case understood as meaning silyl substituents each having, independently of one another, three identical or different radicals which are chosen from the radicals C₁-C₆-alkyl, C₇-C₁₂-aralkyl and substituted or unsubstituted C₆-C₁₀-aryl. By way of example, mention may be made here, for example, of the silyl substituents trimethylsilyl, triethylsilyl, tert-butyldimethylsilyl and tert-butyldiphenylsilyl.

Within the scope of the present invention, the substituents —NR^(8a)R^(9a) to —NR^(8d)R^(9d) are in each case amino substituents which, in each case independently of one another, carry two identical or different, preferably two identical, radicals which are chosen from the abovedescribed radicals C₁-C₆-alkyl, C₇-C₁₂-aralkyl and/or substituted or unsubstituted C₆-C₁₀-aryl. By way of example, mention may be made of the amino substituents: dimethylamino, diethylamino, dibenzylamino, diallylamino, diisopropylamino. Within the scope of the present invention, the radicals R^(8a) and R^(9a) to R^(8d) and R^(9d) may independently of one another, in each case together also form a cyclic hydrocarbon radical having 2 to 8 carbon atoms which can have one or more identical or different heteroatoms chosen from the group O, S, NR^(11a). The radical R^(11a) can here be abovedescribed C₁-C₆-alkyl, C₇-C₁₂-aralkyl and/or substituted or unsubstituted C₆-C₁₀-aryl. Examples of these cyclic substituents R^(8a) and R^(9a) to R^(8d) and R^(9d) which may be mentioned are: piperidinyl, morpholinyl, N-methylpiperazinyl, N-benzylpiperazinyl.

In the substituents —SR^(10a), the radical R^(10a) is an abovedefined C₁-C₆-alkyl, C₇-C₁₂-aralkyl and/or substituted or unsubstituted C₆-C₁₀-aryl, preferably methyl, ethyl, isopropyl, phenyl, benzyl.

Within the scope of the present invention, preferred aromatic or heteroaromatic substituents Ar¹, Ar², Ar³, Ar⁴ are, for example, phenyl, 4-methylphenyl, 2,4,6-trimethylphenyl, naphthyl, 2-fluorophenyl, 4-fluorophenyl, 4-chlorophenyl, 4-bromophenyl, 3-fluorophenyl, 3-chlorophenyl, 3,5-difluorophenyl, 3,5-dichlorophenyl, 2,3,6-trichlorophenyl, 2,4,6-trichlorophenyl, 2-methylphenyl, 4-methylphenyl, 2,4,5-trimethylphenyl, 2,4,6-trimethylphenyl. 2-isopropylphenyl, 4-isopropylphenyl, 4-tert-butylphenyl, 4-n-butylphenyl, 3-trifluoromethylphenyl, 4-trifluoromethylphenyl, 3,5-bis(trifluoromethyl)phenyl, 4-arylphenyl, 3-nitrophenyl, preferably 4-fluorophenyl, 4-chlorophenyl, 3-chlorophenyl, 3,5-dichlorophenyl, 3-trifluoromethylphenyl, 4-trifluoromethylphenyl. Within the scope of a preferred embodiment, the radicals Ar¹, Ar², Ar³, Ar⁴ are identical and are preferably 4-fluorophenyl, 4-chlorophenyl, 3-chlorophenyl, 3,5-dichlorophenyl, 3-trifluoromethylphenyl, 4-trifluoromethylphenyl, particularly preferably phenyl.

According to the invention, the substituents R¹, R²R³, R⁴ in the meta or para position relative to the respective phenolic hydroxy groups may be identical or different, preferably identical, and, in each case independently of one another, are hydrogen and/or an abovementioned C₁-C₆-alkyl, C₁-C₆-perfluoroalkyl, C₁-C₆-alkoxy, C₇-C₁₂-aralkyl, halogen, —SiR^(5b)R^(6b), R^(7b), substituted or unsubstituted C₆-C₁₀-aryl, —NR^(8b)R^(9b), —SR^(10b) and/or —NO₂.

Preferred radicals R¹, R², R³, R⁴ which may be mentioned are: methyl, ethyl, isopropyl, halogen, in particular fluorine and/or chlorine, trifluoromethyl, phenyl, methoxy, nitro. Preferably, the radicals R¹, R², R³, R⁴ are identical and are particularly preferably hydrogen.

The radicals R¹ or R² and/or R³ or R⁴ may, together with the structural element A, also form a cyclic aromatic or nonaromatic cycle. In these cases, the bis(diarylphenol) ligands of the formula (I) to be used according to the invention have a tricyclic basic structure, for example an anthracene basic structure of the formula (X) or basic structures of the type (XI):

Further structural modifications of these tricyclic basic structures, if appropriate also those which have heteroatoms in the basic structure, are known to the person skilled in the art and belong to the group of bis(diarylphenol) ligands which can be used according to the invention.

The structural element A in formula (I) can be a straight-chain or branched and/or cyclic hydrocarbon radical having 1 to 25 carbon atoms, which may be saturated or mono- or polyunsaturated, normally 1 to about 6-fold unsaturated and/or may be partially aromatic. The specified hydrocarbon radicals can, if appropriate, have one or more, generally 1 to 3, identical or different heteroatoms chosen from the group of heteroatoms O, S and NR¹¹ and/or one or more identical or different functional groups chosen from the group of functional groups C(O), S(O) and S(O)₂, and if appropriate carry one or more identical or different substituents chosen from the group of the substituents C₁-C₆-alkyl C₁-C₆-perfluoroalkyl, C₁-C₆-alkoxy, C₁-C₁₀-acyloxy, C₇-C₁₂-aralkyl, halogen, —SiR^(5c)R^(6c)R^(7c), substituted or unsubstituted C₆-C₁₀-aryl, substituted or unsubstituted C₂-C₁₀-hetaryl, —NR^(8c)R^(9c), —SR^(10C), —NO₂, C₁-C₁₂-acyl and C₁-C₁₀-carboxyl.

Preferably, the structural element A in formula (I) is a straight-chain or branched and/or cyclic hydrocarbon radical having 1 to 25, preferably 1 to 15 and particularly preferably 1 to 10, carbon atoms, which may be saturated or mono- to triunsaturated and/or partially aromatic. The preferred hydrocarbon radicals can, if appropriate, have one or more, generally 1 to 3, identical or different heteroatoms chosen from the group of heteroatoms O, S and NR¹¹ and/or one or more C(O) groups and, if appropriate, carry one or more identical or different substituents chosen from the group of substituents C₁-C₆-alkyl, C₁-C₆-perfluoroalkyl, C₁-C₆-alkoxy, C₁-C₁₀-acyloxy, C₇-C₁₂-aralkyl, halogen, substituted or unsubstituted C₆-C₁₀-aryl, —NO₂, C₁-C₁₂-acyl and C₁-C₁₀-carboxyl.

Examples of structural elements A in the formula (I) which may be mentioned without any limiting character are the following structural elements 1 to 44, where the wavy lines in each case mark, as within the scope of the overall present disclosure, the linkage sites to the remainder of the particular ligand structure:

The structural elements 1 to 44 shown can in each case also carry the substituents referred to above and, if appropriate, have further, usually 1 or 2, ethylenic double bonds.

The structural element A can also be an aryl radical having 6 to 15, preferably 6 to 10, carbon atoms, specifically a phenylene, naphthylene or anthracenylene radical, or a heteroaryl radical as defined above having 2 to 15, preferably 3 to 10, carbon atoms.

The specified aryl and heteroaryl radicals may, if appropriate, in each case carry 1 to 5 substituents which are chosen from the group of abovedescribed substituents C₁-C₆-alkyl, C₁-C₆-perfluoroalkyl, C₁-C₆-alkoxy, C₇-C₂-aralkyl, halogen, —SiR^(5d)R^(6d), R^(7d), substituted or unsubstituted C₆-C₁₀-aryl, —NR^(8d), R^(9d), SR¹⁰ and NO₂.

Furthermore, the structural element A can also be a functional group or a heteroatom which is chosen from the group —O—, —S—, —N(R¹¹)—, —S(O)—, —C(O)—, —S(O)₂—, —P(R¹¹)—, —(R¹¹)P(O)—, —OP(O)O—, —OP(O)₂O— and —Si(R¹²)(R¹³)—, where the radicals R¹¹, R¹², R¹³, independently of one another, are in each case an abovedescribed C₁-C₆-alkyl, C₇-C₁₂-aralkyl and/or substituted or unsubstituted C₆-C₁₀-aryl. Within this group, the structural element A is preferably —O—, —S—, —S(O)—, —S(O)₂— or —Si(R¹²)(R¹³)—.

Within the scope of the present invention, the term “ligand in free or complex-bound form” comprises both the free form of the ligand and all conceivable forms which can be converted into the free form under the process conditions. Examples thereof which may be mentioned are alkoxides of the ligand, which are converted to the free form of the ligand by basic hydrolysis.

Within the scope of the present invention, the expression “aqueous base” generally comprises aqueous solutions whose pH is greater than 7. In particular, these are aqueous solutions of alkali metal and alkaline earth metal hydroxides, specifically aqueous solutions of KOH and NaOH.

Within the scope of the present invention, the expression “aluminum-containing reaction product” is a reaction product which comprises at least one compound which comprises aluminum in ionic, covalent or complex-bound form. These are compounds of aluminum as result under the conditions of the method according to the invention from the compounds of the formula (R¹⁴)_(3-p)AlH_(p) (II) or MAlH₄ (III), as defined below, used in the cyclization of citronellal. Within the scope of the present invention, the expression “majority” should be understood as meaning a percentage fraction of the total amount of a compound present which is greater than 50%, preferably greater than 80% and particularly preferably greater than 90%.

Step a):

In step a) of the method according to the invention, the aluminium-containing reaction product from the production of isopulegol by cyclization of citronellal is subjected to distillative separation to give an isopulegol-enriched top product and an isopulegol-depleted bottom product.

In a specific embodiment, step a) uses a solvent with a higher boiling point than that of the isopulegol. In this way, undesired thermal stressing of the bottom contents can be avoided. In particular, the ligands of the formula (I) present therein are not in a form free from solvent while separating off the isopulegol. The higher-boiling solvent can be added to the aluminum-containing reaction product before and/or during distillative separation. Preference is given to using a high-boiling solvent whose boiling point under the conditions of the distillation is above the boiling point of the isopulegol. Preferably, the boiling point of the introduced solvent under the conditions of the distillation is at least 5° C., preferably at least 10° C. and in particular at least 20° C., above the boiling point of the isopulegol.

Preferred higher-boiling solvents which have such a boiling point are, for example, hydrocarbons, such as phenylcyclohexane, benzyltoluene, dibenzyltoluene, 1-methyl-naphthalene and tridecane, 1-decanol, 1,2-propylene carbonate, ethers, such as diethylene glycol dibutyl ether, tetraethylene glycol dimethyl ether and dibenzyl ether, and technical-grade mixtures of these solvents. Particular preference is given to mixtures which comprise phenylcyclohexane as main constituent.

When using at least one higher-boiling solvent, the isopulegol-depleted bottom product in step a) obtained is an organic phase comprising the higher-boiling solvent, the majority of the ligands of the formula (I) and, if appropriate, at least one aluminum-containing compound.

Preferably, distillative separation of isopulegeol in step a) takes place at a bottom temperature of preferably at most 250° C., preferably at most 150° C. and particularly preferably at most 100° C. The lower bottom temperature is usually uncritical and is generally at least 0° C., preferably at least 20° C. To maintain these maximum temperatures, the distillation can, if desired, be carried out under a suitable vacuum.

The pressure in step a) of the method according to the invention is, irrespective of the specific embodiment, generally in a range from 0.1 to 1500 mbar, preferably in a range from 1 to 500 mbar and particularly preferably in a range from 5 to 100 mbar.

Irrespective of the composition of the aluminum-containing reaction product from the cyclization of citronellal and of the use of a higher-boiling solvent, distillative separation of the isopulegol can take place continuously or batchwise, preferably continuously. In one suitable procedure, the higher-boiling solvent is added to the reaction product from the cyclization of citronellal before distillative separation and in the course of the distillation the amount of high-boiling solvent present in the bottom is subsequently kept constant.

For the distillative separation in step a), the customary devices known to the person skilled in the art can be used (see e.g. Sattler, Thermische Trennverfahren [Thermal separation methods], 2nd Edition 1995, Weinheim, p. 135ff; Perry's Chemical Engineers Handbook, 7th Edition 1997, New York, Section 13). These include distillation columns which may be provided with packings, internals etc. The distillation columns used can comprise separation-effective internals, such as separation trays, e.g. perforated trays, bubble-cap trays or valve trays, arranged packings, e.g. sheet-metal or fabric packings, or random beds of packings. The number of plates required in the column(s) used and the reflex ratio are essentially governed by the purity requirements and the relative boiling position of the constituents in the aluminum-containing reaction product from the production of isopulegol by cyclization of citronellal and of the higher-boiling solvent, where the person skilled in the art can ascertain the specific design and operating data by known methods. The distillative separation can, for example, take place in one or more distillation columns coupled together.

Likewise suitable for the distillative separation in step a) are customary evaporators, preferably evaporators with forced circulation, particularly preferably falling-film evaporators.

Depending on additional components which may, if appropriate, be present in the aluminum-containing reaction product from the cyclization of citronellal, the composition of the top product obtained during distillative separation may make it necessary to subject said product, if appropriate, to a further work-up step.

In a specific embodiment of the method according to the invention for working up an aluminum-containing reaction product from the production of isopulegol by cyclizing citronellal, the reaction product additionally comprises a lower-boiling solvent (iii).

Within the scope of the present invention, the expression “lower-boiling solvent (iii)” refers to the boiling point of the isopulegol. Of particular suitability here are those solvents or solvent mixtures which, under the conditions of the distillative separation, have a boiling point which is at least 5° C., preferably 10° C. and in particular 20° C. below that of the isopulegol under the respective conditions.

Within the scope of the present invention, preferred solvents with such a boiling point are inert organic solvents or mixtures thereof, such as, for example, aromatic solvents, e.g. toluene, ethylbenzene or xylene, halogenated solvents, e.g. dichloromethane, dichloroethane or chlorobenzene, aliphatic solvents, e.g. pentane, hexane or cyclohexane, ethers, e.g. tetrahydrofuran, diethyl ether, methyl tert-butyl ether, esters, e.g. ethyl acetate, or dimethylformamide (DMF), dimethyl sulfoxide (DMSO) and the like. Particular preference is given to toluene.

If the aluminum-containing reaction product to be worked up comprises such a low-boiling solvent, then this is removed at least partially from the reaction product in a suitable embodiment prior to the distillative separation of the isopulegol. The lower-boiling solvent is likewise preferably separated off by distillation. Depending on the boiling point of the lower-boiling solvent, the customary abovementioned distillation devices can be used.

In a further suitable embodiment, distillative separation of the aluminum-containing reaction product in step a) takes place to give an isopulegol-enriched top product which at the same time comprises at least some, preferably the majority, of the lower-boiling solvent. In this case, the top product can be subjected to further separation, preferably likewise by distillation.

The separated-off lower-boiling solvent is advantageously returned to the cyclization of the citronellal by using it as solvent. In this way, the method according to the invention requires—apart from top-ups which are required as a result of unavoidable losses—just the single provision of an amount of the lower-boiling solvent.

In a specific embodiment of the method according to the invention for working up an aluminum-containing reaction product from the production of isopulegol by cyclizing citronellal, the reaction product additionally comprises an auxiliary (iv).

Within the scope of the present invention, the term “auxiliary” (iv) refers to compounds which are added during the cyclization of citronellal in order to suppress undesired secondary reactions. Preferably, the auxiliaries (iv) are chosen from organic acids, carboxylic anhydrides, aldehydes, ketones and vinyl ethers.

Specifically, the auxiliaries (iv) are chosen from acids, preferably organic acids. By way of example, organic acids which may be mentioned are: acetic acid, propionic acid, benzoic acid, toluenesulfonic acid, methanesulfonic acid, preferably acetic acid.

In a further specific embodiment of the present invention, the auxiliaries (iv) are chosen from carboxylic anhydrides, aldehydes, ketones and vinyl ethers.

The auxiliaries (iv) of said classes of substance can in each case be present individually or in the form of mixtures in the reaction product to be worked up. Preferred mixtures are those which consist of compounds of one class of substance. The reaction product particularly preferably comprises a single auxiliary.

Preferably, the auxiliaries (iv) present in the reaction product from the cyclization of citronellal are likewise at least partially removed and as far as possible returned to the cyclization of citronellal.

If the auxiliaries (iv) under the conditions of the distillation have a boiling point which is below or only slightly, i.e. less than 30° C., above the boiling point of the isopulegol, these can be largely recovered from the fully reacted mixture by distillation to the extent to which it was not, if appropriate, itself reacted. Depending on the boiling point of the auxiliary, the customary abovementioned distillation devices can be used.

If the auxiliaries (iv) have a boiling point under the conditions of the distillation which is significantly above, i.e. at least 30° C., above the boiling point of the isopulegol, these remain in the bottom product and are, if appropriate, removed in step b) of the method according to the invention if their physical properties allow this.

In a further suitable embodiment, distillative separation of the reaction product in step a) takes place to give an isopulegol-enriched top product which at the same time comprises at least some, preferably the majority, of the auxiliary (iv). If appropriate, this main product can comprise a lower-boiling solvent, as explained above. In this case, the top product can be subjected to further separation, preferably likewise by distillation. The separated-off auxiliary (iv) is, if appropriate together with the lower-boiling solvent, advantageously returned to the cyclization of the citronellal, where it is used, for example, for suppressing undesired secondary reactions. In this way, the method according to the invention requires—apart from top-ups which are required as a result of unavoidable losses—just a single provision of an amount of the auxiliary (iv).

The separating off of isopulegol, the introduction of the higher-boiling solvent and, if appropriate, the separating off of low-boiling components, i.e. the separating off of any solvents present and volatile auxiliaries from the cyclization of citronellal, can be combined in various ways:

In one suitable embodiment, a so-called dividing wall column is used for the distillation, i.e. feed point and a side take-off are located on opposite sides of a dividing wall which extends along a section of the longitudinal expansion of the column. Such distillation columns which comprise a dividing wall are known per se to the person skilled in the art. If side take-off and feed are in the region of the dividing wall, a connection analogous to a Brugma or Petlyuk connection arises. Such distillations using dividing wall columns are described in DE-A-33 02 525 and EP-A-0 804 951, to the entire scope of which reference is hereby made. In this case, a fraction enriched with low-boiling components can be removed as top product, and a stream comprising the majority of isopulegol can be removed as side take-off, for example. The higher-boiling solvent is introduced below the feed point, preferably into the bottom of the column or just above the bottom. A solution of the majority of the ligand of the formula (I) in the higher-boiling solvent is produced as bottom product.

In an alternative embodiment, coupled columns are used for the distillation. This embodiment may be advantageous if the reaction product of the cyclization of citronellal comprises a solvent and/or a volatile auxiliary, as explained in more detail below.

In this case, mixtures of isopulegol and lower- or slightly higher-boiling solvents and/or auxiliary (iv) can form the top product of the first column and in the second column be subjected to separation to give a stream comprising at least the majority of the isopulegol and an isopulegol-depleted stream comprising the lower-boiling solvents and/or auxiliaries of the cyclization.

Streams which can comprise lower-boiling solvents (iii) and auxiliary (iv) of the cyclization can usually be returned to the cyclization without further separation.

The ligands of the formula (I) are produced, if appropriate in the form of their complexes or other derivatives, as bottom product of the first column.

Step b):

In step b) of the method according to the invention, the isopulegol-depleted bottom product is brought into close contact with an aqueous base to give an aluminum-containing aqueous phase and an organic phase comprising the majority of the ligands of the formula (I). Preferred aqueous bases are given above.

Besides the ligand of the formula (I) in free or complex-bound form, the isopulegol-depleted bottom product obtained in step a) can comprise at least one further low-volatile component. These include, for example, higher-boiling solvents added in step a), the reaction products of the aluminum-containing compounds used for the cyclization of citronellal to isopulegol, and, if appropriate, auxiliaries (iv) not separated off in step a). Since aluminum-containing components and/or the auxiliaries (iv) accumulate particularly in the case of a continuous method and have an adverse effect especially on the yield and purity of the separation step c), it is advantageous to remove these compounds as completely as possible. This applies specifically to the aluminum-containing compounds.

The bringing into contact in step b) preferably takes place by extraction. The number of extraction stages is preferably in a range from 1 to 20 stages.

The extractants used are the abovementioned aqueous bases. These expressions are therefore used synonymously within the scope of the present invention.

For the extraction, the isopulegol-depleted bottom product from step a) is brought into close contact with an aqueous base. Separation of the phases gives a phase comprising the majority of the ligand of the formula (I) and an aqueous phase enriched in aluminum-containing compounds. The aqueous phase is then removed. The bringing into contact can take place continuously or batchwise.

For the batchwise procedure the isopulegol-depleted bottom product from step a) and the aqueous extractant are brought into contact with mechanical agitation, e.g. by stirring, in a suitable vessel, the mixture is left to stand for phase separation and one of the phases is removed by expediently removing the denser phase at the bottom of the vessel.

A plurality of batchwise separation operations can be carried out successively in a cascade-like manner, in which case the phase separated off from the aqueous phase and comprising the majority of the ligand of the formula (I) is in each case brought into contact with a fresh portion of the aqueous extractant and/or the aqueous extractant is passed countercurrently.

The extraction preferably takes place continuously. For the continuous extraction procedure, the aqueous extractant and the stream of isopulegol-depleted bottom product from step a) is introduced continuously into suitable apparatuses in a manner analogous to the batchwise variant. At the same time, a discharge of the phase comprising the majority of the ligand of the formula (I) and a discharge of the aqueous phase enriched in aluminum-containing compounds are continuously removed from the apparatus in which the separation of the phases takes place.

The extraction takes place at least in one stage, e.g. in a mixer-separator combination. Suitable mixers are either dynamic or static mixers. Extraction in a plurality of stages takes place, for example, in a plurality of mixer-separators or extraction columns.

In one suitable embodiment, at least one coalescing device is used to improve phase separation. This is preferably chosen from coalescing filters, electrocoalescers and combinations thereof. When using mixer-separator devices for the extraction, the use of coalescing filters, such as candle filters or sand filters, has proven advantageous for improving phase separation. The filter can be installed here directly after the mixer (stirred container) and/or in the organic run-off from the separator. Also preferred for improving phase separation is the use of electrocoalescers. These have proven useful for separating off aqueous foreign phases of up to 5% by mass. The use of coalescing devices is also advantageously suitable in the method according to the invention for separating off finely dispersed aqueous phase from the organic discharge of an extraction column comprising the majority of the ligand of the formula (I).

In one suitable embodiment, the extraction takes place in at least one mixer-separator combination for the extraction of aluminum-containing components from the isopulegol-depleted bottom product from step a). The use of a further mixer-separator combination is particularly advantageous for subsequently reextracting and thus returning to the process fractions of the ligand of the formula (I) or, if appropriate, of the higher-boiling solvent which, if appropriate, with the aluminum-containing compounds to be separated off, partially pass into the extractant.

Under certain circumstances, it may be advantageous to subject the organic phase comprising the majority of ligands of the formula (I) to a drying step before separating off the ligand in step c) or after separating it off. Suitable drying methods are the customary ones known to the person skilled in the art, in particular the adsorption to dehydrating agents, e.g. using a zeolitic molecular sieve.

In an alternative embodiment of the method according to the invention, after bringing the isopulegol-depleted bottom product into contact with the aqueous base, the water is completely or at least partially removed by distillation.

In order to prevent the ligand of the formula (I) from separating off prematurely, specifically by crystallization, at no point during step b) should the solubility product of the ligand in the organic phase be exceeded. This can be carried out through appropriate choice of the temperature and/or the amount and type of added solvents, if appropriate.

Consequently, in a preferred embodiment of the method according to the invention, a discharge of the heated bottom product from step a) is brought into close contact with a heated aqueous base.

Within the scope of the present invention, the expression “heated” refers to a temperature above room temperature and below the respective boiling point temperatures of the aqueous or organic solutions under the reaction conditions in question. In particular, heated refers to a temperature in the range from 25° C. to 150° C., specifically in the range from 70° C. to 100° C.

Depending on the auxiliaries used, if appropriate, in the cyclization of citronellal, the isopulegol-depleted bottom product can, if appropriate, comprise further components not separated off in step a). These are preferably separated off in step b). In this case, the aqueous phase obtained can be subjected to a suitable separation process in order to recover these components, e.g. an auxiliary (iv).

Step c).

In step c) of the method according to the invention, the ligand of the formula (I) is separated off from the organic phase comprising the majority of the ligand obtained in step b) by crystallization, where step c) can be carried out continuously or batchwise. Suitable embodiments of this step are, for example, crystallization and/or complete or at least partial distillative removal of volatile constituents.

In one preferred embodiment of the method according to the invention, the ligand of the formula (I) is separated off by crystallization.

For the crystallization of the ligand of the formula (I), the solubility product of the ligand of the formula (I) in the organic phase from step b) must firstly be exceeded. This can take place, for example, by a cooling process of the organic phase or by (partial) distillative separating off of the solvent. Methods for this purpose are known to the person skilled in the art. For the technical configuration of the method according to the invention, customary cooling crystallizers, evaporating crystallizers, vacuum crystallizers, crystallizing troughs or spray crystallizers, for example, are suitable.

In one preferred embodiment of the method according to the invention, crystallization takes place by cooling the organic phase from step b) of the method. In general, crystallization takes place at a temperature in the range from −50° C. to 100° C., preferably in the range from −20° C. to 50° C. and specifically in a range from 10° C. to 40° C.

This process can be accelerated by adding seed crystals.

The crystalline ligand of the formula (I) can be isolated from the solution, for example, by filtration, flotation, centrifugation or sieving.

The ligand of the formula (I) retained in this way can, if appropriate, be dried by suitable drying methods. Methods for this are known to the person skilled in the art. For example, for the technical configuration of the method, customary roller dryers, disk dryers, chamber dryers, fluidized-bed dryers or radiation dryers may be suitable.

The organic phase depleted in ligand of the formula (I) can again be added to the process before or during step a).

In one suitable embodiment of the method according to the invention, crystallization takes place upon cooling to room temperature from a heated, saturated organic phase obtained in step b).

In a preferred embodiment of the method for working up a reaction product from the production of isopulegol, the ligand of the formula (I) is chosen from bis(diarylphenol) ligands of the formula (I.a)

where Ar¹, Ar², Ar³, Ar⁴, R¹, R², R³, R⁴ and A have the meanings given above.

The ligands of the formula (I.a) likewise have two phenol systems which in each case are substituted in both ortho positions relative to the phenolic hydroxy group by aromatics or heteroaromatics (Ar¹ to Ar⁴) and are joined together via a structural element A and, if appropriate, can also carry further substituents (R¹ to R⁴), the structural element A being joined to the two phenol systems in each case in the para position relative to the phenolic hydroxy group. Here, the radicals Ar¹, Ar², Ar³, Ar⁴, the radicals R¹, R², R³, R⁴ and the structural element A can have the same meanings as specified above for formula (I).

According to the invention, particularly preferred ligands are those in which the aryl radicals Ar¹, Ar², Ar³ and Ar⁴ are identical and have the preferred meanings given above for formula (I). Particularly preferred aryl radicals Ar¹ to Ar⁴ are phenyl, naphthyl, 4-fluorophenyl, 4-chlorophenyl, 3-chlorophenyl, 3,5-dichlorophenyl, 4-methylphenyl, 3-trifluoromethylphenyl, 4-trifluoromethylphenyl, very particularly preferably phenyl.

In the ligands of the formula (I.a) preferred according to the invention, the radicals R¹, R², R³, R⁴ are identical or different, preferably identical, and are preferably: hydrogen, halogen, in particular fluorine or chlorine, methyl, trifluoromethyl, isopropyl, tert-butyl, phenyl, nitro.

The structural element A in formula (I.a) has the meanings given above for formula (I). Preferred structural elements A in formula (I.a) are in particular also the structural elements 1 to 44 which may be substituted in the specified manner.

Particularly preferred ligands are those of the formulae (I.a₁) to (I.a₃), where the specified radicals Ar¹ to Ar⁴, R¹ to R⁴ and R¹⁵ to R¹⁸ preferably have the meanings listed by way of example in the table:

TABLE 1 (I.a₁)

Com- pound Ar¹ Ar² Ar³ Ar⁴ R¹ R² R³ R⁴ R¹⁵ Ia₁-1 Ph Ph Ph Ph H H H H H Ia₁-2 Ph Ph Ph Ph H H H H CH₃ Ia₁-3 Ph Ph Ph Ph H H H H Ph Ia₁-4 Ph Ph Ph Ph H H H H CF₃ Ia₁-5 Ph Ph Ph Ph H H H H CCl₃ Ia₁-6 Ph Ph Ph Ph H H H H 4-Cl-Ph Ia₁-7 Ph Ph Ph Ph H H H H CH₂CH₃ Ia₁-8 Ph Ph Ph Ph H H H H 3-NO₂-Ph Ia₁-9 Ph Ph Ph Ph H H H H

TABLE 2 (I.a₂)

Compound Ar¹ Ar² Ar³ Ar⁴ R¹ R² R³ R⁴ R¹⁶ R¹⁷ Ia₂-1 Ph Ph Ph Ph H H H H CF₃ CF₃ Ia₂-2 Ph Ph Ph Ph H H H H CCl₃ CCl₃ Ia₂-3 Ph Ph Ph Ph H H H H CH₃ CF₃ Ia₂-4 Ph Ph Ph Ph H H H H CH₃ CCl₃ Ia₂-5 Ph Ph Ph Ph H H H H CH₂CH₃ CF₃ Ia₂-6 Ph Ph Ph Ph H H H H CH₃ CH₃ Ia₂-7 Ph Ph Ph Ph H H H H CH₃ C(O)OCH₃ Ia₂-8 Ph Ph Ph Ph H H H H CH₃ C(O)OC₂H₅ Ia₂-9 Ph Ph Ph Ph H H H H —(CH₂)₃— Ia₂-10 Ph Ph Ph Ph H H H H —(CH₂)₄— Ia₂-11 Ph Ph Ph Ph H H H H —(CH₂)₅—

TABLE 3 (I.a₃)

Compound Ar¹ Ar² Ar³ Ar⁴ R¹ R² R³ R⁴ R¹⁸ Ia₃-1 Ph Ph Ph Ph H H H H —(CH₂)₂— Ia₃-2 Ph Ph Ph Ph H H H H

Ia₃-3 Ph Ph Ph Ph H H H H

Ia₃-4 Ph Ph Ph Ph H H H H

Ia₃-5 Ph Ph Ph Ph H H H H

Ia₃-6 Ph Ph Ph Ph H H H H

Ia₃-7 Ph Ph Ph Ph H H H H

Here, in Tables 1-3, Ph is a phenyl radical and C(O) is a carbonyl group within the scope of the present invention. In general, the radicals R¹⁵, R¹⁶ and R¹⁷ can, independently of one another, be an abovedefined C₁-C₆-alkyl, C₁-C₁₀-acyl, C₁-C₁₀-carboxyl or C₆-C₁₀-aryl, where the specified radicals can carry one or more identical or different halogen and/or NO₂ substituents and where the radicals R¹⁶ and R¹⁷ can together also form a cyclic structural element, preferably an alkylene bridge.

The present invention further provides a method for producing isopulegol of the formula (IV)

comprising

-   α) the cyclization of citronellal of the formula (V)

-   -   in the presence of a catalyst which is obtainable by reacting a         bis(diarylphenol) ligand of the formula (I) as defined in claims         1 and/or 10,     -   with an aluminum compound of the formula (II),

(R¹⁴)_(3-p), AlH_(p)  (II)

-   -   where     -   Al is aluminum,     -   R¹⁴ is a branched or unbranched alkyl radical having 1 to 5         carbon atoms and     -   p is 0 or an integer from 1 to 3,     -   and/or     -   with an aluminum compound of the formula (III),

MAlH₄  (III)

-   -   where     -   Al is aluminum and     -   M is lithium, sodium or potassium,

-   β) the recovery of the bis(diarylphenol) ligand of the formula (I)     after the reaction has taken place by     -   a) subjecting the aluminum-containing reaction product obtained         in step α) to distillative separation to obtain an         isopulegol-enriched top product and an isopulegol-depleted         bottom product,     -   b) bringing the isopulegol-depleted bottom product into close         contact with an aqueous base to give an aluminum-containing         aqueous phase and an organic phase comprising the majority of         the ligands of the formula (I) and     -   c) separating off the ligand of the formula (I) from the organic         phase.

In a specific embodiment of the method according to the invention for producing isopulegol, the ligand of the formula (I) is separated off by crystallization.

With regard to the preferred embodiments of the method according to the invention for working up a reaction product from the production of isopulegol by cyclization of citronellal, and for the preferred ligands of the formula (I), reference is made to the abovementioned preferred embodiments in their entirety.

The bis(diarylphenol) ligands of the formulae (I) and (I.a) which can be used for producing the bis(diarylphenoxy)aluminum compounds used according to the invention can be prepared easily by methods known per se to the person skilled in the art. Compounds of structure type (I.a₁) are obtained, for example, by reacting the corresponding bis-ortho-arylphenols with an aldehyde R¹⁵CHO in the presence of a Lewis acid, for example AlCl₃, as described, inter alia, by Z. Y. Wang, A. S. Hay in Synthesis 1989, 471-472 or in U.S. Pat. No. 3,739,035. Ligands of structure type (I.a₂) are, for example, accessible by reacting the corresponding bis-ortho-arylphenols with a suitable ketone of the formula R¹⁶C(O)R¹⁷, as described, for example, in U.S. Pat. No. 3,739,035. Ligands of structure type (I.a₃) are, for example, accessible by Friedel-Crafts acylation of the corresponding phenols or O-protected phenols with dicarboxylic acid chlorides, as described, for example, by F. F. Blicke et al. in J. Am. Chem. Soc. 1938, 60, 2283-2285; CH 350461 or by G. Maier et al. in Chem. Bern 1985, 118, 704-721. Another way of producing ligands of structure type (Ia₃) also consists in the Friedel-Crafts alkylation of the corresponding phenols with tertiary diols, as described, for example, in DE-A 25 34 558, or with dihalides, as described, for example, by J. Zavada, in Collect. Czech. Chem. Commun., 1976, 41, 1777-1790.

The bis(diarylphenoxy)aluminum compounds used according to the invention are obtained, for example, by reacting the abovedescribed bis(diarylphenol) ligands of the formulae (I) or (I.a) with an aluminum compound of the formula (II)

(R¹⁴)_(3-p)AlH_(p)  (II).

Here, R¹⁴ is a branched or unbranched alkyl radical having 1 to 5 carbon atoms, such as, for example, methyl, ethyl, propyl, isopropyl, butyl, isobutyl, pentyl, isopentyl or neopentyl. The index p is 0 or an integer from 1 to 3. Preferably, the index p is 1 or 0, particularly preferably 0. Preferred compounds of the formula (II) are, for example, trimethylaluminum, triethylaluminum, diisobutylalumninum hydride, particularly preferably trimethylaluminum and triethylaluminum.

Alternatively to this, the bis(diarylphenoxy)aluminum compounds used according to the invention can also be obtained by reacting the abovedescribed bis(diarylphenol) ligands of the formulae (I) or (I.a) with an aluminum compound of the formula (III)

MAlH₄  (III),

where M is lithium, sodium or potassium. Consequently, of suitability for producing the bis(diarylphenoxy)aluminum compounds used according to the invention by reacting the abovedescribed bis(diarylphenol) ligands of the formulae (I) or (I.a) are also lithium aluminum hydride, sodium aluminum hydride and potassium aluminum hydride, and mixtures thereof. Moreover, mixtures of the specified compounds of the formulae (II) and (II) are also suitable for producing bis(diarylphenoxy)aluminum compounds used according to the invention by reaction with the abovedescribed bis(diarylphenol) ligands of the formulae (I) or (I.a).

The reaction is advantageously carried out so that one of the abovedescribed bis(diarylphenol) ligands of the formulae (I) or (I.a) is brought into contact with a compound of the formula (II) or (III). The reaction is advantageously carried out in an inert organic solvent, such as, for example, toluene, cyclohexane, dichloromethane, xylene, ethylbenzene, chlorobenzene, tetrahydrofuran, diethyl ether, methyl tert-butyl ether, ethyl acetate, pentane, hexane, dichloroethane, dimethylformamide (DMF), dimethyl sulfoxide (DMSO) and the like, the use of predried or anhydrous solvents being regarded as particularly advantageous. The reaction is usually carried out at temperatures in the range from about −100° C. to about 100° C., preferably at about −50° C. to about 50° C., particularly preferably at about −30° C. to about 30° C.

During the production of the bis(diarylphenoxy)aluminum compounds according to the invention, the phenolic hydroxy groups of the bis(diarylphenol) ligands of the formulae (I) or (I.a) used react with the compound or compounds of the formulae (II) and (III). Theoretically, each aluminum atom can react with 1 to 3 phenolic hydroxy groups. On account of the steric properties or requirements of the bis(diarylphenol) ligands of the formulae (I) or (I.a) used, this results in the formation of higher molecular weight structures such as linear structures or networks.

Here, the molar ratio of the bis(diarylphenol) ligands of the formulae (I) or (I.a) used to the compounds of the formula (II) and/or (III) used is advantageously chosen such that the amount of incompletely reacted compounds of the formulae (II) and/or (III) is as low as possible. Preferably, the specified ratio is chosen so that, after the bis(diarylphenol) ligands of the formulae (I) or (I.a) have been brought into contact with the compound or the compounds of the formulae (II) and (III), incompletely reacted compound of the formula (II) and/or (III) is no longer present. Taking into consideration the cost aspect, it is advisable to keep the excess of the ligands of the formulae (I) or (I.a) used low. Particular preference is given to using bis(diarylphenol) ligands of the formulae (I) or (I.a) and the compounds of the formulae (II) and/or (III) in a molar ratio of from about 4:1 to about 1:1, very particularly preferably from about 3:1 to about 1.5:1 and most preferably in the molar ratio of about 1.5:1.

Within the scope of a preferred embodiment of the present invention, the production of the bis(diarylphenoxy)aluminum compounds used according to the invention involves initially introducing, depending on the solubility, an about 0.001 to about 1 molar solution of the chosen ligand of the formula (I) or (I.a) into a suitable organic solvent, for example toluene, at a temperature of from about −10 to about 30° C., and adding an aluminum compound of the formula (II) and/or (III), preferably in the form of a solution, for example a solution of trimethyl- or triethylaluminum in toluene.

The reaction between the ligands of the formula (I) or (I.a) used and the aluminum compounds of the formulae (II) and/or (III) usually takes place rapidly and is mostly complete after about 10 min to about 2 h, often after about 1 h, depending on the reaction conditions chosen. When using more unreactive reactants, it may be advantageous to temporarily increase the temperature of the reaction mixture.

Depending on the reaction conditions chosen, in particular with regard to the solubility of the ligands of the formula (I) or (I.a) to be reacted and of the aluminum compound of the formula (II) and/or (III) in the chosen solvents, the concentrations and the reaction temperatures, the bis(diarylphenoxy)aluminum compounds according to the invention are obtained in the form of a solid, a suspension or a solution in the solvent or solvent mixture used. The bis(diarylphenoxy)aluminum compounds used according to the invention obtained in this way can be further used in the form obtained in each case or are separated off and freed from the solvents used.

Isolation can take place here by methods which appear to be advantageous and are known to the person skilled in the art. Preferably, the isolation, storage and further treatment of the bis(diarylphenoxy)aluminum compounds used according to the invention are carried out with extensive exclusion of oxygen and moisture.

To carry out the method according to the invention for producing isopulegol, the procedure advantageously involves firstly preparing a solution of the bis(diarylphenoxy)aluminum compounds used according to the invention in a suitable solvent, as described above. The racemic or nonracemic citronellal to be cyclized is then added according to the invention to this solution. The citronellal can here be added as it is or in the form of a solution, advantageously in one of the abovementioned suitable solvents. Within the scope of a preferred embodiment of the method according to the invention, a solution of the chosen ligand of the formulae (I) or (I.a) in toluene is firstly prepared and then, advantageously with stirring, the chosen aluminum compound of the formula (II) and/or (III), preferably trimethylaluminum or triethylaluminum in toluenic solution, is added.

A suitable starting material for carrying out the cyclization method according to the invention is citronellal, which can be produced by any method. Preference is given to using citronellal which has a purity of about 90 to about 99.9% by weight, particularly preferably from about 95 to about 99.9% by weight.

The addition of the citronellal to be cyclized advantageously takes place at temperatures in the range from about −40° C. to about 40° C., preferably in the range from about −20° C. to about 20° C. For this, the prepared solution of the bis(diarylphenoxy)aluminum compound used according to the invention is advantageously cooled to a temperature in this range, e.g. to a temperature in the range from −10° C. to 10° C., and prechilled citronellal or a prechilled solution of citronellal is added.

The addition of the citronellal or of the solution thereof can be undertaken such that either the whole amount is added at once or it is added in portions or else continuously to the prepared catalyst solution. Suitable solvents in turn are the abovementioned solvents, in particular toluene. Preferably, the citronellal to be cyclized is used as it is, i.e. without the further addition of solvents. When using a solvent, the total amount of solvent (for catalyst production and for carrying out the cyclization reaction) is advantageously chosen so that the volume-based ratio of citronellal to be reacted to solvent is about 2:1 to about 1:20, preferably from about 1.5:1 to about 1:10.

The quantitative ratio between the citronellal to be reacted and the amount of bis(diarylphenoxy)aluminum compound employed according to the invention used is determined by the amount of compounds of the formula (I) or (I.a) and of the formula (II) and/or (III) used for producing the same, i.e. by the quantitative ratio of ligand used to aluminum compound of the formula (II) and/or (III) used.

According to the invention, the amount of citronellal to be reacted relative to the amount of aluminum compound of the formula (II) and/or (III) used is chosen such that the molar ratio is about 5:1 to about 1000:1, preferably about 10:1 to about 500:1, particularly preferably about 50:1 to about 200:1.

Irrespective of this, the ratio between ligand of the formula (I) or (I.a) used and the aluminum compound of the formula (II) and/or (III) used can be varied within the limits specified above for producing the bis(diarylphenoxy)aluminum compound according to the invention.

The cyclization of citronellal to isopulegol generally takes place rapidly, depending on the choice of reactants and reaction conditions, and is usually largely complete after about 0.5 to about 10 h, often after about 5 h. Reaction progress can be easily monitored by methods known per se to the person skilled in the art, for example by chromatographic, specifically gas chromatographic, methods or else HPLC methods.

Within the scope of a preferred embodiment of the method according to the invention, the cyclization of citronellal to isopulegol is carried out in the presence of an auxiliary (iv), for example an acid, preferably an organic acid. By way of example, organic acids which can be used advantageously are: acetic acid, propionic acid, benzoic acid, toluenesulfonic acid, methanesulfonic acid, preferably acetic acid. The specified acids are advantageously used in an amount of from about 0.5 to about 10% by weight, based on the amount of citronellal to be reacted. Advantageously, they are added to the reaction mixture together with the citronellal, e.g. in the form of a mixture.

In a particularly preferred embodiment, the method according to the invention for producing isopulegol by cyclizing citronellal is carried out in the presence of at least one auxiliary (iv) which is chosen from carboxylic anhydrides, aldehydes, ketones and vinyl ethers.

The auxiliaries (iv) of the specified classes of substance can in each case be used individually or in the form of mixtures with one another. In the case of mixtures, preference is given to using those which consist of compounds of one class of substance. Particular preference is given to using individual compounds. By using the specified compounds as described below, it is generally possible to largely suppress the formation of undesired by-products.

Within the scope of a preferred embodiment, the cyclization of citronellal is carried out in the presence of a carboxylic anhydride of the formula (VI)

where the radicals R²⁰ and R^(20′) may be identical or different, preferably identical, and are a branched or unbranched C₁-C₁₂-alkyl radical or C₇-C₁₂-aralkyl radical or a C₆-C₁₀-aryl radical, where the specified radicals may in each case have one or more, generally 1 to about 3, identical or different substituents chosen from the group OR^(10e), SR^(10f)NR^(8e)R^(9e) and halogen and where R²⁰ and R^(20′) can together also form a 5- to 8-membered ring which can have one or more ethylenic double bonds and one or more identical or different heteroatoms chosen from the group O, S and NR^(11b) and where R^(10e), R^(10f), R^(8e), R^(9e) and R^(11b) can have the meanings given above for R¹¹.

Within the scope of a further preferred embodiment, the cyclization of citronellal is carried out in the presence of an aldehyde of the formula (VII)

where the radical R²¹ is a branched or unbranched C₁-C₁₂-alkyl radical or C₇-C₁₂-aralkyl radical or a C₆-C₁₀-aryl radical, where the specified radicals can in each case have one or more, preferably 1 to 3, identical or different substituents chosen from the group OR^(10e), SR^(10f)NR^(8e)R^(9e) and halogen, where R^(10e), R^(10f), R^(8e) and R^(9e) can have the meanings given above for R¹¹.

Within the scope of a further preferred embodiment, cyclization of citronellal is carried out in the presence of a ketone of the formula (VIII)

where the radicals R²² and R²³ may in each case be identical or different and are a branched or unbranched C₁-C₁₂-alkyl radical or C₇-C₁₂-aralkyl radical or a C₆-C₁₀-aryl radical or a C₁-C₆-alkoxycarbonyl radical, where the specified radicals can in each case have one or more, preferably 1 to 3, identical or different substituents chosen from the group OR^(10e), SR^(10f)N^(8e)R^(9e) and halogen, and where R²² and R²³ can together also form a 5- to 8-membered ring which can have one or more ethylenic double bonds and one or more identical or different heteroatoms chosen from the group O, S, NR^(11b) and where R^(10e), R^(10f), R^(8e), R^(9e) and R^(11b) can have the meanings given above for R¹¹.

As an alternative to the abovementioned carbonyl compounds, it is also possible to use vinyl ethers of the general formula (IX)

within the scope of the method according to the invention, where the radicals R²⁴, R²⁵, R²⁶ and R²⁷, independently of one another, may in each case be identical or different and are a branched or unbranched C₁-C₁₂-alkyl radical or C₇-C₁₂-aralkyl radical or a C₆-C₁₀-aryl radical, where the specified radicals can in each case have one or more, preferably 1 to 3, identical or different substituents chosen from oxo, OR^(10e), SR^(10f)NR^(8e)R^(9e) and halogen and where R²⁵ and R²⁶ can together also form a 5- to 8-membered ring which can have one or more ethylenic double bonds and one or more, usually 1 or 2, identical or different heteroatoms chosen from the group O, S, NR^(11b) and where R^(10e), R^(10f), R^(8e), R^(9e) and R^(11b) can have the meanings given above for R¹¹.

C₁-C₁₂-Alkyl here is C₁-C₆-alkyl as described above and, moreover, for example heptyl, octyl, nonyl, decyl, undecyl or dodecyl. In the cases where two alkyl radicals together form a ring, alkyl radicals are also understood as meaning alkylenyl radicals. C₇-C₁₂-Aralkyl radicals and C₆-C₁₀-aryl radicals can, by way of example, have the meanings given above. By way of example, C₁-C₆-alkoxycarbonyl radicals which may be mentioned are: methoxycarbonyl, ethoxycarbonyl, propoxycarbonyl and isopropoxycarbonyl, preferably methoxycarbonyl and ethoxycarbonyl.

Within the scope of a preferred embodiment of the method according to the invention, the cyclization of citronellal is carried out in the presence of a carboxylic anhydride of the formula (VI), where the radicals R²⁰ and R^(20′) are identical and are a branched or unbranched C₁-C₁₂-alkyl radical or C₇-C₁₂-aralkyl radical or a C₆-C₁₀-aryl radical, and where R²⁰ and R^(20′) can together also form a 5- to 8-membered ring which can have one or more ethylenic double bonds and one or more identical or different heteroatoms chosen from the group OR^(10e), SR^(10f)NR^(11b), and R^(10e), R^(10f) and R^(11b) can, independently of one another, have the meanings given above for R¹¹.

Particular preference is given to using those carboxylic anhydrides in which the radicals R²⁰ and R^(20′) are identical and are a branched or unbranched C₁-C₁₂-alkyl radical or a C₆-C₁₀-aryl radical. By way of example, carboxylic anhydrides to be used particularly preferably according to the invention are: acetic anhydride, propionic anhydride, pivalic anhydride and benzoic anhydride.

Aldehydes of the formula (VII) which can likewise be used preferably according to the invention are, by way of example, acetaldehyde, propionaldehyde and chloral (trichloroacetaldehyde).

If, within the scope of a further preferred embodiment, the cyclization of citronellal is carried out in the presence of a ketone of the formula (VIII), it is advantageous to use those with an activated, i.e. electron-deficient, carbonyl function. By way of example, mention may be made of the following ketones which are particularly suitable for use within the scope of the method according to the invention: 1,1,1-trifluoroacetone, 1,1,1-trifluoroacetophenone, hexafluoroacetone, methyl pyruvate and ethyl pyruvate.

Vinyl ethers of the formula (IX) which can likewise be used with preference according to the invention are, for example: methyl vinyl ether, ethyl vinyl ether, isobutyl vinyl ether and 3,4-dihydro-2H-pyran.

The specified classes of compound can be used equally with good success within the scope of this preferred embodiment of the method according to the invention. With regard to practical aspects such as, for example, a higher reaction rate, the use of aldehydes and/or electron-deficient ketones has proven to be advantageous.

The amount of carboxylic anhydride, aldehyde, ketone and/or vinyl ether to be used according to the invention can be varied within wide limits and is governed by the type of substance used and the degree of purity or the presence of impurities which are as yet not more precisely identified. Usually, the specified compounds and mixtures thereof are used in an amount of from about 0.01 mol % to about 5 mol %, preferably from about 0.1 mol % to about 2 mol %, based on the amount of citronellal used.

The type and manner of the procedure, for example the configuration of reactors or the order in which individual reactants are added, are not subject to particular requirements provided a procedure with extensive exclusion of oxygen and water is ensured.

To carry out the method according to the invention within the scope of this preferred embodiment, the procedure advantageously involves firstly providing a solution of the bis(diarylphenoxy)aluminum compound to be used according to the invention in a suitable solvent as described above. Then, according to the invention, a mixture of the racemic or nonracemic citronellal to be cyclized with the chosen carboxylic anhydride, the aldehyde, the activated ketone and/or the vinyl ether is preferably added to this solution. Alternatively thereto, it is possible, for example, to also admix the solution of the bis(diarylphenoxy)aluminium compound to be used according to the invention firstly with the carboxylic anhydride, of appropriate chosen in each case, the aldehyde, the ketone and/or the vinyl ether, and to afterwards add the citronellal to be cyclized.

It has proven to be advantageous to meter in the citronellal or the mixture of citronellal with the chosen compound to the catalyst solution or to the reaction mixture within a period of from about 30 min to about 6 h, preferably within about 2 h to about 4 b. The citronellal can here be added as such or in the form of a solution, advantageously in one of the abovementioned suitable solvents. Within the scope of an again preferred embodiment of the method according to the invention, a solution of the chosen ligand of the formulae (I) or (I.a) in toluene is firstly provided, and then the chosen aluminum compound of the formula (II) and/or (III), preferably trimethylaluminum or triethylaluminum in toluenic solution is added, expediently with stirring.

The addition of the citronellal to be cyclized or the mixture of citronellal with the chosen carboxylic anhydride, aldehyde, activated ketone and/or the vinyl ether takes place within the scope of this embodiment advantageously at temperatures in the range from about −40° C. to about 40° C., preferably in the range from about −20° C. to about 20° C. For this, the prepared solution or suspension of the bis(diarylphenoxy)aluminum compound according to the invention is advantageously cooled to a temperature within this range, e.g. to a temperature in the range from −10° C. to 10° C., and the other reactants are added in precooled form.

The addition of the mixture of citronellal and the chosen further compound can be undertaken so that either the total amount of citronellal is added in one go or it is added in portions or continuously to the prepared catalyst solution. Suitable solvents are in turn preferably the abovementioned solvents, in particular toluene. Preference is given to using the citronellal to be cyclized in the form of a mixture with the chosen carboxylic anhydride, aldehyde, activated ketone and/or vinyl ether without the further addition of solvents. When using a solvent, the total amount of solvent is advantageously chosen so that the volume-based ratio of citronellal to be reacted to the solvent is about 1:1 to about 1:20, preferably from about 1:1 to about 1:10.

It has been found that some of the catalyst complex is usually deactivated during the reaction. This is attributed, inter alia, to ligand exchange processes between the bis(diarylphenol) ligands of the formula used in each case of the bis(diarylphenoxy)aluminum compounds used and the isopulegol which forms as a result of cyclization. The deactivated form of the catalyst is, depending on the choice of solvent used, soluble in the reaction mixture, usually in contrast to the active polymeric catalyst.

In one preferred embodiment, the deactivated part of the catalyst can be separated off together with the other reaction mixture by simple physical separation methods (e.g. by filtering off or centrifuging the catalyst which is still active). The retained, still active part of the catalyst can, if desired, be supplemented with a flesh catalyst and be reused without appreciable loss in activity, preferably within the scope of a further cyclization reaction according to the invention of citronellal to isopulegol.

Alternatively, the amount of catalyst used can be chosen so that the total catalyst complex used is deactivated and thus soluble in the course of or at the end of the cyclization reaction according to the invention, something which is recognizable from a clear reaction mixture. Here, it is advantageously notable that in this case, on account of the abovedescribed ligand exchange processes, the bis(diarylphenol) ligand of the formula (I) used in each case is released without separate hydrolysis being carried out.

Surprisingly, it has been found that isopulegol can be distilled off from the aluminum-containing reaction products of the cyclization of citronellal without prior hydrolysis of the bis(diarylphenoxy)aluminum compounds used in each case as catalyst (if appropriate following distillative removal of a solvent used and/or additionally used auxiliaries) in high purities. As a rule, no recognizable undesired or troublesome by-products form in the distillation bottom. In a specific embodiment, a suitable, inert, high-boiling solvent is added before or during the distillative separation in step a). A solution of the ligand of the formula (I) in the heated high-boiling component used in each case is then obtained in the distillation bottom.

As already mentioned, the method according to the invention is equally suitable for cyclizing racemic and nonracemic, i.e. optically active, citronellal to give racemic and nonracemic isopulegol.

In a preferred embodiment, the method according to the invention thus serves for producing optically active isopulegol of the formula (IV.a)

by cyclization of active citronellal of the formula (V.a)

where (*) in each case refers to an asymmetric carbon atom.

The method according to the invention serves in particular for producing L-(−)-isopulegol by cyclization of D-(+)-citronellal.

The racemic or nonracemic isopulegol produced in this way is a valuable intermediate for producing racemic or nonracemic menthol, one of the most significant fragrances or aromas worldwide. Menthol can be obtained from isopulegol by methods of hydrogenation known per se to the person skilled in the art, specifically catalytic hydrogenation over suitable transition metal catalysts, as described, for example, in Pickard et al., J. Chem. Soc. 1920, 1253; Ohloff et al., Chem. Ber. 1962, 95, 1400; Pavia et al., Bull. Soc. Chim. Fr. 1981, 24, Otsuka et al., Synthesis 1991, 665 or in EP 1 053 974 A. Here, if the chosen reaction conditions are suitable, the relative or absolute configuration of the isopulegol used is largely retained, and in many cases is completely retained.

The present invention therefore further provides a method of producing menthol comprising the steps:

A) production of isopulegol of the formula (IV) by a method according to the invention B) hydrogenation of the ethylenic double bond of the isopulegol obtained in this way.

In a preferred embodiment, this method serves for producing optically active menthol, specifically for producing L-(−)-menthol from optically active L-(−)-isopulegol.

With regard to the preferred embodiments of the method according to the invention for producing isopulegol, reference is made to the abovementioned preferences in their entirety.

The examples below serve to illustrate the present invention without having any limiting character.

EXAMPLE 1 Process for recovering 1,1-bis(2,6-diphenylphenol)-1-trifluoromethylethane (Ia₂-3)

Gas chromatographic analyses (GC) were carried out according to the following method: 50 m CP-WAX, ID.: 0.32 mm, FD.: 1.2 ym; 80° C., 3° C./min −200° C., 15° C./min to 250° C.; t_(R) (phenylcyclohexane): 30.7, t_(R) (isopulegol): 26.3; t_(R) (citronellal): 21.8.

The following HPLC method was used: CC250/4 Nucleodur C18 Gravity, 5 ym; C: water—0.05% H₃PO₄; D: acetonitrile 20:80; exit: 93 bar, 25° C.; t_(R) (phenylcyclohexane): 10.5; t_(R) (isopulegol): 3.3; t_(R) (ligand (Ia₂-3)): 14.0. Concentrations of the reaction products obtained in the distillation bottom and in the mother liquor (in each case in % by weight) were determined analytically by GC and HPLC using an internal standard.

1.a) Cyclization of Citronellal

1,1-Bis(2,6-diphenylphenol)-1-trifluoromethylethane (Ia₂-3) (461 g, 0.785 mmol) in anhydrous toluene (7.2 l) was initially introduced into a jacketed glass reactor with stirrer. A solution of triethylaluminum in toluene (445 ml, 400 mmol, 12% AlEt₃ in toluene) was added to the clear solution of the ligand at room temperature. The solution was stirred for 1 h at 25° C. The resulting catalyst suspension was cooled to 0° C. and admixed over a period of 3 h with a mixture of citronellal (6697 g, 43 mol) and methyl pyruvate (33.6 g, 329 mmol). When addition was complete, the reaction mixture was after-stirred for 3 h at 0° C. and for a further 2 h at 110° C. Toluene was separated off under reduced pressure. An isopulegol crude product was then separated off by distillation as top product with the addition of phenylcyclohexane (2770 g). 3584 g of a bottom product were obtained.

1.b) Isolation of the Ligand (Ia₂-3)

3564 g of the bottom product from the cyclization of citronellal in the presence of a (bis(diarylphenoxy))aluminum catalyst comprising phenylcyclohexane (69.9% by weight), isopulegol (3.05% by weight), citronellal (0.16% by weight) and citronellol (3.05% by weight) were initially introduced into a jacketed reactor with stirrer and reflux condenser at a temperature of 90° C. 1792 g of a heated 2% strength aqueous NaOH solution were added to the heated solution. After stirring for one hour at 90° C., 1777 g of the aqueous phase were separated off from the organic phase. The remaining water from the organic phase was distilled off at 120° C. and 10 mbar. The hydrolyzed bottom product was cooled to 25° C. over the course of 12 hours. The resulting suspension of the ligand of the formula (Ia₂-3) was filtered and the ligand of the formula (Ia₂-3) obtained in this way was freed from volatile constituents at 3 mbar and 95° C. The ligand of the formula (Ia₂-3) was isolated as white solid with a yield of 282 g and a purity of 95%. According to HPLC analysis, the mother liquor (3130 g) comprised phenylcyclohexane (72.3% by weight), isopulegol (6.8% by weight) and ligand of the formula (Ia₂-3) (4.9% by weight). This demonstrates that the ligands used according to the invention are suitable in an advantageous manner for a continuous work-up. By contrast, when using the ligands described in EP-A 1 225 163, separation of the phases is not ensured in every case since these have a greater tendency to form stable emulsions.

EXAMPLE 2 Continuous process for recovering 1,1-bis(2,6-diphenylphenol)-1-trifluoromethylethane (Ia₂-3) Analysis

Gas chromatographic analyses were carried out according to the following method: 50 m CP-WAX, ID).: 0.32 mm, FD.: 1.2 ym; 80° C., 3° C./min −200° C., 15° C./min to 250° C.; t_(R) (citronellal): 20.7; t_(R) (isopulegol): 24.7; t_(R) (phenylcyclohexane): 29.3; t_(R) (citronellol): 31.7; t_(R) (citronellol citronellate): 48.2; t_(R) (isopulegyl citronellate): 49.5.

2.a) Cyclization of Citronellal During Continuous Work-Up

A solution of triethylaluminum in toluene (15% strength, 85 ml, 0.096 mol) is added at 20° C. over the course of about 10 min to a clear solution of 1,1-bis(2,6-diphenylphenol)-1-trifluoromethylethane (Ia₂-3) (114 g, 0.195 mol) in toluene (anhydrous, 1800 g) in a jacketed glass reactor with stirrer. The solution is then stirred at 20° C. for 1 h. The resulting catalyst suspension is transferred to a further jacketed glass reactor with stirrer, cooled to 0° C. and admixed over a period of 3 h with a mixture of D-citronellal (1620 g, 10.3 mol) and methyl pyruvate (8.1 g). When addition is complete, the reaction solution is stirred at 0° C. until a content of <10 GC area % of D-citronellal is reached, warmed to 10° C. and stirred for a further 2 h at this temperature. Subsequently, the reaction solution is firstly transferred to a buffer container.

The reaction solution is passed to a plate column (15 plates, DN 50) continuously at a feed rate of 300 g/h. Toluene is removed from the column at a head pressure of about 100 mbar at a side take-off at the 10th plate in the enriching section, where the bottom temperature is about 120° C. and the temperature of the side take-off and of the top of the column are 45° C. The low-boiling components are eliminated from the reaction solution at the top of this column.

A discharge of the bottom product from the plate column is fed continuously (120 to 140 g/h) into the center of a packed column (DN 50×120 cm, laboratory fabric packing, Sulzer DX). With the continuous addition of phenylcyclohexane (70 to 90 g/h) into the bottom of this packed column, L-isopulegol is distilled off as top product at a bottom temperature of 110° C. and a head pressure of 10 mbar. L-isopulegol is isolated in a yield of 1625 g and in a purity of 93%.

2.b) Isolation of the Ligand (Ia₂-3) During Continuous Procedure

A discharge of the distillation bottom of the packed column is fed continuously (100 to 120 g/h) to a mixer-settler apparatus heated to 95° C. and consisting of two cascaded 250 ml stirred containers and a 150 ml phase separator. In the first 250 ml stirred container, the discharge of the distillation bottom of the packed column is admixed continuously with a feed of 2% strength sodium hydroxide solution (50 to 60 g/h). A discharge (150 to 180 g/h) of the mixed phase from the first stirred container is transferred to the 150 ml phase separator via the other 250 ml stirred container. In the phase separator, the continuous separation of the phases takes place at a temperature of from 90 to 95° C. The height of the phase separation layer is regulated here with the help of conductivity measurements.

The discharge of the organic phase from the phase separator is collected continuously (100 to 120 g/h) in a further stirred container heated to 40 to 50° C. and left to crystallize prior to isolation of the ligand (Ia₂-3). A discharge of the aqueous phase from the phase separator is continuously pumped off.

The crystallized ligand (Ia₂-3) is filtered batchwise through a pressure filter at a nitrogen pressure of 4 bar. The filtercake is then washed with phenylcyclohexane. The washed ligand (106 g; HPLC % by weight: ligand 77%; phenylcyclohexane 22%) is dissolved in toluene and further used for preparing the catalyst in step 2.a). The filtrate (919 g; % by weight according to GC; phenylcyclohexane 66%; L-isopulegol 5%; citronellol 6.1%; isopulegyl citronellate 4.3%; citronellyl citronellate 3.6%; % by weight according to HPLC: ligand 3.1%) is returned to the packed column described under 2.b). 

1-19. (canceled)
 20. A method for working up an aluminum-containing reaction product from the production of isopulegol by cyclizing citronellal, comprising i) isopulegol; and ii) at least one ligand of the formula (I),

wherein Ar¹, Ar², Ar³, and Ar⁴ are, independently of one another, a C₆-C₁₅-aryl radical or a C₂-C₁₅-heteroaryl radical, wherein said C₆-C₁₅-aryl radical and said C₂-C₁₅-heteroaryl radical is optionally substituted with up to 7 identical or different substituents selected from the group consisting of C₁-C₆-alkyl, C₁-C₆-perfluoroalkyl, C₁-C₆-alkoxy, C₇-C₁₂-aralkyl, halogen, SiR^(5a)R^(6a)R^(7a), optionally substituted C₆-C₁₀-aryl, NR^(8a)R^(9a), SR^(10a), and NO₂; R¹, R², R³, and R⁴ are, independently of one another, hydrogen, C₁-C₆-alkyl, C₁-C₆-perfluoroalkyl, C₁-C₆-alkoxy, C₇-C₁₂-aralkyl, halogen, SiR^(5b)R^(6b)R^(7b), optionally substituted C₆-C₁₀-aryl, NR^(8b)R^(9b), SR^(10b), or NO₂, wherein R¹ or R² and/or R³ or R⁴ together with A optionally define an aromatic or nonaromatic cycle; and A is a straight-chain or branched and/or cyclic hydrocarbon radical having up to 25 carbon atoms, wherein said straight-chain or branched and/or cyclic hydrocarbon radical is optionally saturated or mono- or polyunsaturated and/or partially aromatic, optionally has one or more identical or different heteroatoms selected from the group consisting of O, S, and NR¹¹ and/or one or more identical or different functional groups selected from the group consisting of C(O), S(O), and S(O)₂, and is optionally substituted with one or more identical or different substituents selected from the group consisting of C₁-C₆-alkyl, C₁-C₆-perfluoroalkyl, C₁-C₆-alkoxy, C₁-C₁₀-acyloxy, C₇-C₁₂-aralkyl, halogen, —SiR^(5c)R^(6c)R^(7c), optionally substituted C₆-C₁₀-aryl, optionally substituted C₂-C₁₀-hetaryl, NR^(8c)R^(9c), SR^(10c), NO₂, C₁-C₁₂-acyl, and C₁-C₁₀-carboxyl, or is a C₆-C₁₅-aryl radical or a C₂-C₁₅-heteroaryl radical, wherein said C₆-C₁₅-aryl radical and said C₂-C₁₅-heteroaryl radical is optionally substituted with up to 5 substituents selected from the group consisting of C₁-C₆-alkyl, C₁-C₆-perfluoroalkyl, C₁-C₆-alkoxy, C₇-C₁₂-aralkyl, halogen, SiR^(5d)R^(6d)R^(7d), optionally substituted C₆-C₁₀-aryl, NR^(8d)R^(9d), SR^(10d), and NO₂, or is a functional group or a heteroatom selected from the group consisting of —O—, —S—, —N(R¹¹)—, —S(O)—, —C(O)—, —S(O)₂—, —P(R¹¹)—, —(R¹¹)P(O)—, and —Si(R¹²R¹³); wherein R^(5a), R^(6a), R^(7a), R^(8a), R^(9a), R^(10a), R^(5b), R^(6b), R^(7b), R^(8b), R^(9b), R^(10b), R^(5c), R^(6c), R^(7c), R^(8c), R^(9c), R^(10c), R^(5d), R^(6d), R^(7d), R^(8d), R^(9d), R^(10d), R¹¹, R¹², and R¹³ are in each case independently of one another selected from the group consisting of C₁-C₆-alkyl, C₇-C₁₂-aralkyl, and/or optionally substituted C₆-C₁₀-aryl, and wherein R^(8a) and R^(9a), R^(8b) and R^(9b), R^(8c) and R^(9c), R^(8d) and R^(9d), independently of one another, in each case optionally define a cyclic hydrocarbon radical having 2 to 8 carbon atoms which optionally has one or more identical or different heteroatoms selected from the group consisting of O, S, and NR^(11a), wherein R^(11a) is as defined as R¹¹; in free and/or complex-bound form; said method comprising: a) subjecting the reaction product to distillative separation to obtain an isopulegol-enriched top product and an isopulegol-depleted bottom product; b) bringing said isopulegol-depleted bottom product into close contact with an aqueous base to give an aluminum-containing aqueous phase and an organic phase comprising the majority of the ligands of formula (I); c) separating off the ligands of formula (I) from said organic phase.
 21. The method of claim 20, wherein said ligand of formula (I) is separated off from the organic phase in c) by crystallization.
 22. The method of claim 20, wherein said aluminum-containing reaction product from the production of isopulegol by cyclizing citronellal additionally comprises a lower-boiling solvent (iii).
 23. The method of claim 20, wherein said aluminum-containing reaction product from the production of isopulegol by cyclizing citronellal additionally comprises an auxiliary (iv).
 24. The method of claim 23, where said auxiliary (iv) is selected from the group consisting of organic acids, carboxylic anhydrides, aldehydes, ketones, and vinyl ethers.
 25. The method of claim 20, wherein, prior to the distillative separation in a), any solvent present and/or auxiliaries from the cyclization are first separated off from said reaction product.
 26. The method of claim 20, wherein, prior to and/or during the distillative separation in a), said reaction product is admixed with a solvent whose boiling point, under the distillation conditions, is at least 10° C. higher than the boiling point of isopulegol.
 27. The method of claim 20, wherein at least one of a), b), and c) is operated continuously.
 28. The method of claim 26, wherein the addition of the higher-boiling solvent takes place during a).
 29. The method of claim 20, wherein a heated discharge of the bottom product from a) is brought into close contact with a heated aqueous base and then the majority of the ligand is isolated from the organic phase.
 30. The method of claim 20, wherein the ligand of formula (I) is a bis(diarylphenol) ligand of formula (I.a)


31. A method for producing isopulegol of formula (IV)

comprising α) cyclizing citronellal of formula (V)

in the presence of a catalyst obtained by reacting a bis(diarylphenol) ligand of formula (I)

wherein Ar¹, Ar², Ar³, and Ar⁴ are, independently of one another, a C₆-C₁₅-aryl radical or a C₂-C₁₅-heteroaryl radical, wherein said C₆-C₁₅-aryl radical and said C₂-C₁₅-heteroaryl radical is optionally substituted with up to 7 identical or different substituents selected from the group consisting of C₁-C₆-alkyl, C₁-C₆-perfluoroalkyl, C₁-C₆-alkoxy, C₇-C₁₂-aralkyl, halogen, SiR^(5a)R^(6a)R^(7a), optionally substituted C₆-C₁₀-aryl, NR^(8a)R^(9a), SR^(10a), and NO₂; R¹, R², R³, and R⁴ are, independently of one another, hydrogen, C₁-C₆-allyl, C₁-C₆-perfluoroalkyl, C₁-C₆-alkoxy, C₇-C₁₂-aralkyl, halogen, SiR^(5b)R^(6b)R^(7b), optionally substituted C₆-C₁₀-aryl, NR^(8b)R^(9b), SR^(10b), or NO₂, wherein R¹ or R² and/or R³ or R⁴ together with A optionally define an aromatic or nonaromatic cycle; and A is a straight-chain or branched and/or cyclic hydrocarbon radical having up to 25 carbon atoms, wherein said straight-chain or branched and/or cyclic hydrocarbon radical is optionally saturated or mono- or polyunsaturated and/or partially aromatic, optionally has one or more identical or different heteroatoms selected from the group consisting of O, S, and NR¹¹ and/or one or more identical or different functional groups selected from the group consisting of C(O), S(O), and S(O)₂, and is optionally substituted with one or more identical or different substituents selected from the group consisting of C₁-C₆-alkyl, C₁-C₆-perfluoroalkyl, C₁-C₆-alkoxy, C₁-C₁₀-acyloxy, C₇-C₁₂-aralkyl, halogen, —SiR^(5c)R^(6c)R^(7c), optionally substituted C₆-C₁₀-aryl, optionally substituted C₂-C₁₀-hetaryl, NR^(8c), R^(9c), SR^(10c), NO₂, C₁-C₁₂-acyl, and C₁-C₁₀-carboxyl, or is a C₆-C₁₅-aryl radical or a C₂-C₁₅-heteroaryl radical, wherein said C₆-C₁₅-aryl radical and said C₂-C₁₅-heteroaryl radical is optionally substituted with up to 5 substituents selected from the group consisting of C₁-C₆-alkyl, C₁-C₆-perfluoroalkyl, C₁-C₆-alkoxy, C₇-C₁₂-aralkyl, halogen, SiR^(5d)R^(6d)R^(7d), optionally substituted C₆-C₁₀-aryl, NR^(8d)R^(9d), SR^(10d), and NO₂, or is a functional group or a heteroatom selected from the group consisting of —O—, —S—, —N(R¹¹)—, —S(O)—, —C(O)—, —S(O)₂—, —P(R¹¹)—, —(R¹¹)P(O)—, and —Si(R¹²R¹³); wherein R^(5a), R^(6a), R^(7a), R^(8a), R^(9a), R^(10a), R^(5b), R^(6b), R^(7b), R^(8b), R^(9b), R^(10b), R^(5c), R^(6c), R^(7c), R^(8c), R^(9c), R^(10c), R^(5d), R^(6d), R^(7d), R^(8d), R^(9d), R^(10d), R¹¹, R¹², and R¹³ are in each case independently of one another selected from the group consisting of C₁-C₆-alkyl, C₇-C₁₂-aralkyl, and/or optionally substituted C₆-C₁₀-aryl, and wherein R^(8a) and R^(9a), R^(8b) and R^(9b), R^(8c) and R^(9c), R^(8d) and R^(9d), independently of one another, in each case optionally define a cyclic hydrocarbon radical having 2 to 8 carbon atoms which optionally has one or more identical or different heteroatoms selected from the group consisting of O, S, and NR^(11a), wherein R^(11a) is as defined as R¹¹; with an aluminum compound of formula (II): (R¹⁴)_(3-p)AlH_(p)  (II) wherein Al is aluminum; R¹⁴ is a branched or unbranched alkyl radical having up to 5 carbon atoms; and p is 0 or an integer from 1 to 3; and/or with an aluminum compound of the formula (III): MAlH₄  (III) wherein Al is aluminum; and M is lithium, sodium, or potassium; and β) recovering the bis(diarylphenol) ligand of formula (I) after the reaction has taken place by a) subjecting the reaction product obtained in α) to distillative separation to obtain an isopulegol-enriched top product and an isopulegol-depleted bottom product; b) bringing the isopulegol-depleted bottom product into close contact with an aqueous base to give an aluminum-containing aqueous phase and an organic phase comprising the majority of the ligands of formula (I); and c) separating off the ligand of formula (I) from the organic phase.
 32. The method of claim 31, wherein the ligand of formula (I) is separated off from the organic phase in c) by crystallization.
 33. The method of claim 31, wherein said aluminum compound of formula (II) is selected from the group consisting of trimethylaluminum and triethylaluminum.
 34. The method of claim 31, wherein the isopulegol of formula (IV) produced is optically active isopulegol of formula (IV.a)

wherein the citronellal of formula (V) cyclized in α) is optically active citronellal of formula (V.a)

wherein (*), in each instance, indicates an asymmetric carbon atom.
 35. The method of claim 34, wherein the optically active isopulegol of formula (IV.a) is L-(−)-ispulegol and the optically active citronellal of formula (V.a) is D-(+)-citronellal.
 36. A method for producing menthol) comprising: A) producing isopulegol of formula (IV) according to the method of claim 31; and B) hydrogenating the ethylenic double bond of the isopulegol of formula (IV) obtained in A).
 37. A method for producing optically active methol, comprising: A) producing optically active isopulegol of formula (IV.a) according to the method of claim 34; and B) hydrogenating the ethylenic double bond of the optically active isopulegol of formula (IV.a) obtained in A).
 38. A method for producing L-(−)-menthol, comprising: A) producing L-(−)-isopulegol according to the method of claim 35; and B) hydrogenating the ethylenic double bond of the L-(−)-isopulegol obtained in A). 